The present invention relates to Direct Sequence Spread Spectrum based wireless communication systems, and deals specifically with multipath channel characterization.
An example of a wireless communication system wherein multipath is encountered is a Wireless Local Area Network (WLAN). With recent advancements in wireless and mobile communications, there has been an increase in the popularity of WLANs as a substitute for the more conventional wired LANs. WLANs are being envisioned for use by enterprises and organizations with the aim of obviating the excess baggage that accompanies a wired LAN, such as plugs and wires. Further, installation of WLANs is easier and less time consuming as compared to the installation of wired LANs. Moreover, WLANs offer significant advantages to enterprise workers in terms of mobility and roaming. The workers may access the Internet and their organization's intranet from public access points such as airports, hotels and conference centres, using laptops, PDAs and other handheld equipment. Thus, the workers may stay connected and carry out their work even when they are on the move.
The Institute of Electrical and Electronics Engineers (IEEE) has proposed the 802.11 family of standards for the implementation of WLANs. The 802.11 family of standards includes the IEEE 802.11 standard and the IEEE 802.11b standard.
The IEEE 802.11 standard provides data transmission rates of 1 to 2 Mbps (Megabits per second) in the 2.4 GHz band. The IEEE 802.11b standard, also known commonly as ‘Wi-Fi’, is an extension of the 802.11 standard and provides data transmission rates of up to 11 Mbps in the 2.4 GHz band. The IEEE 802.11b standard is emerging as one of the more dominant standards for the commercial implementation of WLANs. As already mentioned, 802.11b is an extension of 802.11 and provides data transmission rates up to 11 Mbps, with a fallback to 5.5, 2 and 1 Mbps. 802.11 provides two types of physical layer specifications. These are Direct Sequence Spread Spectrum (DSSS) and Frequency-Hopped Spread Spectrum (FHSS). Both DSSS and FHSS physical layers support data rates of 1 Mbps as well as 2 Mbps. However, only DSSS is suitable for data rates of 5.5 Mbps and 11 Mbps, and is therefore used in the physical layer of 802.11b.
In such DSSS systems, the information bit sequence to be transmitted is first encoded into either Differential Binary Phase Shift Keying (DBPSK) symbols or Differential Quaternary Phase Shift Keying (DQPSK) symbols. Thereafter, each symbol is spread so as to cover a wider bandwidth. The spreading is achieved by multiplying the symbol with a spreading sequence. The spread symbol stream is then modulated onto a carrier frequency; the modulated signal is transmitted wirelessly. The transmitted signal is received and demodulated by a receiver and the information bit sequence is obtained.
In the wireless transmission of a DSSS signal, the signal typically travels through a number of different paths in the air. These multiple paths are caused by the reflection of the signal by various objects in the environment. The signals received on each of these paths are affected by delay, attenuation and a random phase introduced by these paths. The signal received at the receiver is the superposition of the signals received on each of these paths. For example, if there are L paths, the received signal is the sum of the L signals received on each of the L paths.
The presence of multipath in a wireless channel introduces Inter-Symbol Interference (ISI) in the received signal. The ISI is due to the received signal being a superposition of the signals received on each of the paths, and the signals received on each of the paths having undergone different delays.
Since a wireless multipath channel is a physical channel and is therefore causal i.e. the output of the channel at any time instant is not dependent on the input to the channel at any succeeding time instances, the ISI introduced in a signal passing through such a channel is only due to the symbols that have already passed through the channel. This ISI is referred to as postcursor ISI. The other type of ISI, referred to as precursor ISI, refers to the ISI caused by future symbols.
The ISI caused due to transmission of a signal through a multipath wireless channel can be removed or cancelled by processing the received signal with an equalizer, a filter that is designed to counter the ISI. This process is referred to as equalization and several techniques exist in the art for this.
Among the several possible equalization schemes, decision feedback equalization is chosen for implementation in the underlying WLAN system in view of its attractive features. A Decision Feedback Equalizer (DFE) consists of a feedforward transversal filter and a feedback transversal filter. The objective of the feedforward transversal filter is to minimize the distortion caused by the precursor ISI, and that of feedback transversal filter is to remove the postcursor ISI. If a channel is characterized in such a way that all the ISI is made up of post cursor ISI, the feedforward filter is not needed; a feedback transversal filter will remove the postcursor ISI. With the channel characterization as above, the feedback transversal filter forms the DFE. When the feedback transversal filter is implemented as a tapped delay line filter, the tap coefficients used in the implementation are known once the channel is characterized.
In reality, a multipath channel may be specular or diffused. A specular multipath channel comprises specular paths wherein each path is completely characterized by a single delay, a single attenuation coefficient and a single random phase. A diffused multipath channel comprises one or more non-specular paths. However, when a band-limited multipath channel is considered, the effective response is continuous in the time domain. Its sampled version may be viewed as a specular multi-path channel. This specular multipath channel may be modeled as a tapped delay line Finite-duration Impulse Response (FIR) filter with differential path delays equal to the tap spacings and the sample values as the tap values. The characterization of such a channel as a causal channel depends on the detection of the arrival time of the earliest path.
In DSSS based WLAN systems, a conventional correlation-based approach is used to detect the arrival times of different paths. In this approach, the received signal is sampled and the cross correlations are computed between the samples of the received signal and the reference spreading sequence for various time lags. The time lag for which the correlation magnitude is maximum is taken as the time of arrival of the earliest path, and also as the starting instant of the channel impulse response. Subsequently, the channel is completely characterized using a tapped delay line, as mentioned above.
However, the conventional approach suffers from the following drawback. If the energy in the earliest path is low, the time lag for which the correlation magnitude is maximized does not correspond to the time of arrival of the earliest path. Consequently, the starting instant of the channel impulse response may be taken to be later than the actual value. This results in precursor ISI, which may not be effectively removed regardless of how complex an equalizer is used. As a result of this, the Packet Error Rate (PER) increases and may go beyond the acceptable levels defined in the IEEE 802.11b standard.
From the above, it is clear that there is a need for a multipath channel characterization technique that estimates the arrival time of the earliest path, regardless of the energy in the earliest path.